1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus, such as a copying machine, a printer, and a fax.
2. Description of the Related Art
Hitherto, a photosensitive member disposed in an electrophotographic image forming apparatus generally has a photosensitive member made up of a charge generation layer and a charge transport layer.
When a print start signal is input, the photosensitive member is driven in a certain direction to start rotation. By applying a bias to a charging apparatus with respect to the surface of the photosensitive member, the surface of the photosensitive member is charged to a certain potential (hereinafter referred to as a “charging step”).
The surface potential of the photosensitive member at that time is called a dark area potential VD. Onto the surface of the photosensitive member which is charged to the VD, a laser beam or an LED beam is irradiated under on/off control in accordance with a signal from a controller (hereinafter referred to as an “exposure step”).
In an area of the surface of the photosensitive member which has been exposed, a potential is changed due to the exposure step and an electrostatic latent image having a different potential from that in the surroundings is formed on the surface of the photosensitive member. In the following description, the potential in the area where the electrostatic latent image is formed with the exposure is called a bright area potential VL.
A development voltage is applied to a developing apparatus which is disposed to face the photosensitive member, whereby charged toner is supplied from the developing apparatus to the electrostatic latent image formed on the surface of the photosensitive member. As a result, the electrostatic latent image is developed as a toner image on the surface of the photosensitive member (hereinafter referred to as a “developing step”). In the following description, the development voltage applied to the developing apparatus in the developing step is denoted by Vdev.
The toner image developed on the surface of the photosensitive member is brought into contact with a transfer material with the rotation of the photosensitive member and is transferred to the transfer material (hereinafter referred to as a “transfer step”). In the transfer step, the toner image is transferred to the transfer material by feeding the transfer material to pass between the photosensitive member and a transfer member, e.g., a transfer roller that is arranged adjacent to the photosensitive member and is rotated at substantially the same speed as the photosensitive member in the same direction as the rotating direction of the photosensitive member at the position where the photosensitive member and the transfer roller are opposed to each other. More specifically, the toner image is transferred from the photosensitive member to the transfer material by applying a bias with a polarity being opposite to that of the toner to the transfer member and by feeding the transfer material to pass between the photosensitive member and the transfer member in that state.
Even when the bias applied to the charging apparatus in the charging step is held constant and the exposure conditions are held constant in the exposure step, the VL is varied in some cases with repetition of image formation. In one case, residual charges are generated in the photosensitive member with the exposure, thus varying the VL during the image formation. In another case, the temperature of the photosensitive member is raised during the rotation due to sliding frictions of the photosensitive member with respect to a charging member and a cleaning member, and heat radiated from an exposure member, a fuser, etc., thus varying the VL.
In other words, when the VL is varied due to the exposure step of the photosensitive member and the temperature rise thereof, development contrast defined by the difference between Vdev and VL is changed. The change of the development contrast leads to a change in amount of toner coated on the photosensitive member and eventually causes a variation of image density on the transfer material. In the following description, the development contrast is denoted by Vcont.
With the view of stabilizing the image density, an image forming apparatus has been proposed so far in which the VL of a photosensitive member is detected by a sensor in advance and image formation conditions, e.g., an amount of supplied toner, are controlled depending on the detection result (see U.S. Pat. No. 6,339,441).
Because of the necessity of additionally installing the sensor to detect the VL of the photosensitive member, however, the proposed apparatus has the problem of increasing the cost and the size of a main unit.
Also, an image forming apparatus is proposed in which the number of rotations of the photosensitive member, which are performed prior to the exposure step for charge-cancelling and charging on the surface of the photosensitive member, is selected based on the temperature and the humidity around the photosensitive member, thereby suppressing a variation of image density when the same image is formed in large number (see Japanese Patent Laid-Open No. 2005-300745).
However, when the number of rotations of the photosensitive member is increased based on the temperature and the humidity around the photosensitive member, an overall printing speed is reduced and productivity of the image forming apparatus is deteriorated.
In view of the above-mentioned problem, an image forming apparatus is proposed in which the VL of a photosensitive member is estimated from the temperature around the photosensitive member, an image formation time, and an image formation stop time, and in which image formation conditions are controlled depending on the estimated result (see Japanese Patent Laid-Open No. 2002-258550).
However, it is confirmed that the VL is varied depending on not only the temperature of the photosensitive member, but also the absolute humidity of an atmosphere environment around the photosensitive member and the image formation time (time during which the main unit is driven). Further, it is confirmed that the variation of VL appears as not only an increase of its absolute value, but also a decrease thereof.
Nevertheless, the known technique disclosed in Japanese Patent Laid-Open No. 2002-258550 does not take into consideration the absolute humidity of the atmosphere environment around the photosensitive member and the image formation time, and it also does not suppose a possibility that the variation of VL occurs as both of an increase of VL and a decrease of VL. For that reason, the known technique cannot estimate the variation of VL with high accuracy.
Thus, the above-described known image forming apparatus cannot obtain an image in stable density by estimating the variation of VL with high accuracy. Herein, a phenomenon that the absolute value of VL is increased with the image formation time in spite of setting conditions in the charging step and the exposure step constant is called a VL-up. Also, a phenomenon that the absolute value of VL is decreased with the image formation time is called a VL-down.
A process of generation of the VL-up and the VL-down with the image formation time will be described below with reference to FIGS. 2 and 3A-3F. FIG. 2 is a conceptual view representing the surface potential of the photosensitive member, and FIGS. 3A-3F are each a chart representing the VL variation with the lapse of the image formation time or the image formation stop time (FIG. 3D).
As shown in FIG. 2, the difference between Vdev and VL, i.e., (Vdev-VL), provides Vcont. The larger Vcont, the larger is the amount of toner developed on the photosensitive member and the higher is image density.
The VL-up means a phenomenon that the VL is varied in the direction of an arrow A in FIG. 2 (i.e., the direction in which the absolute value is increased), whereby the Vcont is decreased and the image density is reduced. On the other hand, the VL-down means a phenomenon that the VL is varied in the direction of an arrow B in FIG. 2 (i.e., the direction in which the absolute value is reduced), whereby the Vcont is enlarged and the image density is increased.
A description is first made of the phenomenon of the VL-up. In an L/L environment (low-temperature and low-humidity environment), e.g., an environment of 15° C. and 10% RH, the phenomenon of the VL-up occurs with the lapse of the image formation time, as shown in FIG. 3A, even when the image formation is continuously performed just on several sheets.
Further, it is confirmed that, in an environment where the atmosphere around the photosensitive member has lower absolute humidity, an increase rate of VL per unit time becomes larger. In other words, the lower the absolute humidity of the atmosphere around the photosensitive member, the more significantly appears the phenomenon of the VL-up.
In addition, the VL-up is affected by the time during which the photosensitive member has been held stopped before the start of the image formation (i.e., the image formation stop time) such that the increase amount of VL becomes larger at a longer image formation stop time.
For example, when the image formation stop time is long, the VL is increased up to V1 as shown in FIG. 3A. However, when the image formation stop time is short, the VL is increased just to V2 lower than V1 as shown in FIG. 3B.
Such a phenomenon of the VL-up is primarily attributable to the fact that the number of residual charges in the photosensitive layer is increased due to the exposure on the photosensitive member during the image formation. Stated another way, in an environment where the absolute humidity of the atmosphere environment around the photosensitive member is low, the resistance of any layer in the photosensitive layer is so increased that movement and injection of charges within the photosensitive layer are hard to smoothly occur, and the number of residual charges in the photosensitive layer is increased. Hence the VL-up is resulted.
The residual charges generated with the image formation are gradually drained to the ground through the photosensitive layer when the image formation is ended and stopped. As the image formation stop time is prolonged, the number of residual charges generated during the preceding image formation is reduced, thus resulting in a state where the residual charges are more apt to accumulate in the next image formation. Accordingly, as the image formation stop time is prolonged, the influence of the VL-up appears more significantly and the increase amount of VL becomes larger when the next image formation is performed.
A description is next made of the phenomenon of the VL-down. In an environment other than low-temperature and low-humidity, e.g., an environment of 23° C. and 50% RH, the phenomenon of the VL-down occurs with the lapse of the image formation time, as shown in FIG. 3C, when the image formation is continuously performed.
On the other hand, the VL having been reduced with the VL-down shows a greater tendency to restore to the original VL as the time during which the image formation is not performed after the image formation (i.e., the image formation stop time) is prolonged.
For example, when the VL in the preceding image formation is reduced to V4 due to the VL-down with the preceding image formation as shown in FIG. 3C, the initial VL in the next image formation shows a value closer to the original VL, i.e., V3, at a longer image formation stop time, as shown in FIG. 3D.
Such a phenomenon of the VL-down is primarily attributable to the fact that the number of residual charges in the photosensitive layer is reduced. Stated another way, the cause of the VL-down resides in that, because the image formation raises the temperature of the photosensitive member and reduces the resistance of the photosensitive layer, the residual charges trapped in the photosensitive layer is moved externally of the photosensitive member.
The temperature rise of the photosensitive member with the lapse of the image formation time is primarily caused by sliding frictions of the photosensitive member with contact members, such as the developing member, the charging member and the cleaning member, and heat radiated from the exposure member, the fuser, etc.
Further, based the above-described experiment results, it is confirmed that the temperature of the photosensitive member can be accurately estimated from the temperature of the atmosphere environment around the photosensitive member, which also causes the temperature rise of the photosensitive member, the image formation time, and the image formation stop time.
Additionally, the above-described phenomena of the VL-up and the VL-down appear either one or both of them depending on the temperature of the atmosphere environment around the photosensitive member and the absolute humidity of the atmosphere environment.
For example, in an environment where the absolute humidity is low, the increase amount of VL due to the VL-up is very large so that the influence of the VL-down does not appear and only the influence of the VL-up significantly appears in many cases. On the other hand, in an environment where the absolute humidity is high, because the VL-up is hard to occur, the influence of the VL-down significantly appears in many cases.
Further, in some environment, the VL-up and the VL-down often occur simultaneously to cause such a phenomenon that, as shown in FIG. 3E, the VL is initially increased and is gradually reduced thereafter.
In another environment, as shown in FIG. 3F, there may cause a phenomenon that the VL is initially reduced and is gradually increased thereafter.
Thus, the following findings are confirmed. The VL-up can be estimated based on the absolute humidity, the temperature, the photosensitive member stop time, the photosensitive member rotation time. Also, the VL-down can be estimated based on the temperature, the photosensitive member stop time, and the photosensitive member rotation time without employing the absolute humidity. Those estimations of the VL-up and the VL-down are described later.
As still another finding, it is confirmed that when the absolute humidity has a high value, the VL-up is not generated and the VL can be accurately estimated by taking into account only the VL-down.